No Newton, no Principia. That much is clear. But did Newton do it alone? He was naturally exposed to the ideas of such predecessors as Descartes and Galileo and such contemporaries as Leibniz and Huygens. That this collective influenced Newton is reflected in his own writing, “If I have seen further it is by standing on the shoulders of giants.” But the larger question regarding the Principia remains. Did Newton do it alone? The answer: not entirely.
Motion and change in motion
Motion and especially change in motion, thanks to Galileo’s work, remained the central focus in science during the 17th century, and the need to resolve these concepts, especially as they pertained to planetary motion, energized many coffeehouse discussions. Rising to the top of these discussions were the concepts of action-at-a-distance, circular motion, and the relation between the two.
1665-66 annus mirabilis
While to many, action-at-a-distance was impossible, toNewton, it wasn’t. Indeed, Newton embraced this concept when he began developing his theory of force during his annus mirabilis (miracle year). This was one of the most famous periods in science and it began in 1665 when Isaac Newton (1642-1727), seeking to get as far away from the Great Plague of London as possible, departed Cambridge University, where he was a student, with all of his books to his family home in the countryside. In his one year of isolation, Newton “voyag[ed] through strange seas of thought alone”  and uncovered the logic and laws of many different phenomena in nature. He was all of 24 years old at the time! [What a great example of Cal Newport’s Deep Work!]
The challenge was circular motion
For Newton, though, circular motion remained covered. Prior to 1679, Newton, along with many others, incorrectly viewed circular motion as an equilibrium between two opposing forces, an inward gravitational force that pulls a circling body toward the center and an apparent outward force that pushes a circling body away from the center. These are referred to as “centripetal”–center seeking–and “centrifugal”–center fleeing–forces, respectively.
1679 – Robert Hooke re-frames circular motion for Newton
But Newton’s mistaken view changed in 1679 when Robert Hooke (1635-1703) properly re-framed the issue. In his letters to Newton, Hooke proposed that planetary orbits follow motions caused by a central attraction continually diverting a straight-line inertial motion into a closed orbit. To Hooke, a single unbalanced force is at work in circular motion, specifically an inverse-square attraction of the sun for the planets, which leads to acceleration as opposed to the non-acceleration involved with Newton’s equilibrium view. Frustrated with the inability of his equilibrium view to describe nature, Newton immediately latched onto Hooke’s concept as the critical missing piece to his evolving philosophy.
Thank goodness for Hooke’s shoulders!
Without the insight provided by Hooke, Newton’s Principia probably would not have happened. It was Hooke who came along and sliced away the confusion by “exposing the basic dynamic factors [of circular motion] with striking clarity.”  It was Hooke who corrected the misconceptions about circular motion. It was Hooke who properly framed the problem. It was Hooke’s conceptual insight and mentoring that “freed Newton from the inhibiting concept of an equilibrium in circular motion.”  With his newfound clarity, Newton let go of the concept of the fictitious centrifugal force, embraced the concept of his newly created centripetal force (Universal Gravitation) pulling toward the center, and so changed the world of science. The year 1679 was a crucial turning point in Newton’s intellectual life and Hooke was the cause.
Why not Hooke?
Given all this, why Newton and not Hooke? Why didn’t Hooke author the Principia? Why all the acclaim to Newton? The short answer, according to science historian Richard Westfall, is that the bright idea is overrated when compared to the demonstrated theory. While Hooke did indeed provide a critical insight to Newton, the overall problem of motion, including a fundamental understanding of Universal Gravitation, remained unsolved, and the reason was that no one, including Hooke, could work out the math. Well, no one except Newton. You see, of Newton’s many annus mirabilis breakthroughs, one of the most impactful was his invention of calculus, and it was his insightful use of calculus that enabled him to quantify time-varying parameters, such as instantaneous rates-of-change.
Unfortunately, Newton had intentionally kept these breakthrough ideas of 1665-66 away from the public, a result of his “paralyzing fear of exposing his thoughts.”  They remained in his private notebooks, sitting on the sidelines, a tool waiting to be used.
1687 – The Principia
It wasn’t until 20 years after his miracle year that Newton finally sat down and created his famous Philosophiae Naturalis Principia Mathematica, since shortened to the Principia. Published in 1687 by the 45-yr-old and eventually hailed by the scientific community as a masterpiece, the Principia presented the foundation of a new physics of motion that we now call Classical Mechanics based on his Laws of Motion and Universal Gravitation.
Let’s not forget Halley’s shoulders!
But why then? What happened to trigger Newton’s massive undertaking? Here we meet the other person critical to the creation of the Principia, namely Edmund Halley (1656-1742). Halley recognized that Newton had something vital to share with the science community; Halley recognized Newton’s genius. And so it was Halley who travelled to Newton in 1684 to call him forward to solve the yet unsolved problems of motion and change in motion.
Thank goodness for Halley! He is one of the heroes in this story. What would have happened had he not been present? He was responsible for lighting the fire within Newton. And he did it with skill, approaching Newton with ego-stroking flattery as reflected by his even more direct, follow-up request in 1687: “You will do your self the honour of perfecting scientifically what all past ages have but blindly groped after.” 
And so the furnace was lit; “a fever possessed [Newton], like none since the plague years.”  Through Halley’s gentle but firm push, Newton shifted his intense focus away from his other pursuits–Newton was Professor of Mathematics at Cambridge at the time–and towards the cosmos. Newton drew upon his volume of unpublished annus mirabilis work and his later Hooke-inspired insights and pursued–slowly, methodically–the answer. And when it was all done, the Principia was born.
Clearly, no Hooke, no Halley, no Principia. But even more clearly, no Newton, no Principia.
The landscape has been so totally changed, the ways of thinking have been so deeply affected, that it is very hard to get hold of what it was like before. It is very hard to realize how total a change in outlook [Newton] produced – Hermann Bondi 
Both Joseph-Louis Lagrange and Pierre-Simon, Marquis de Laplace regretted that there was only one fundamental law of the universe, the law of universal gravitation, and that Newton had lived before them, foreclosing them from the glory of its discovery – I. Bernard Cohen and Richard S. Westfall 
 Wordsworth, William. 1850. The Prelude. Book Third. Residence at Cambridge, Lines 58-63. “And from my pillow, looking forth by light/Of moon or favouring stars, I could behold/The antechapel where the statue stood/Of Newton with his prism and silent face/The marble index of a mind for ever/Voyaging through strange seas of Thought, alone.”
 Westfall, Richard S. 1971. Force in Newton’s Physics: The Science of Dynamics in the Seventeenth Century. American Elsevier, New York, p. 426.
 Westfall, p. 433.
 Cohen, I. Bernard, and Richard S. Westfall, eds. 1995. Newton: Texts, Backgrounds, Commentaries. 1st ed. A Norton Critical Edition. New York, NY: W.W. Norton. p. 314. Referenced to John Maynard Keynes, “Newton, the Man,” in The Royal Society Newton Tercentenary Celebrations (1947). “[Newton’s] deepest instincts were occult, esoteric, semantic—with profound shrinking from the world, a paralyzing fear of exposing his thoughts, his beliefs, his discoveries in all nakedness to the inspection and criticism of the world.”
 Gleick, James. 2003. Isaac Newton. 1st ed. New York: Pantheon Books, p. 129.
 Gleick, p. 124.
 Bondi, Hermann. 1988. “Newton and the Twentieth Century—A Personal View.” In Let Newton Bel A New Perspective on His Life and Works (Editors: R. Flood, J. Fauvel, M. Shortland, R. Wilson).
 Cohen and Westfall, p. xiv-xv.
Thank you for reading my post. I go into much greater detail about the life and accomplishments of Sir Isaac Newton (1642-1727) in my book, Block by Block – The Historical and Theoretical Foundations of Thermodynamics. Energy came to be viewed through the Classical Mechanics paradigm created by Newton in the Principia. An excellent account of Newton’s work can be found in Richard Westfall’s Force in Newton’s Physics: The Science of Dynamics in the Seventeenth Century. American Elsevier, New York.